Interpretive Summary: The widespread use of the herbicide glyphosate and glyphosate-resistant (GR) crops represents a major advancement in effective weed management in agricultural systems. Although GR crops are developed to resist the growth-inhibitory effects that cause glyphosate to kill weeds, several reports suggest glyphosate may detrimentally effect certain processes including nutrient uptake and photosynthesis in GR crops. This potential of reduction in plant growth and grain yield combined with concerns about possible future limited water availability for crop production led us to investigate the impacts of glyphosate on the ability of GR soybean to efficiently obtain water for optimum growth. We conducted greenhouse studies with GR soybean growing in nutrient solution (hydroponics) that received different rates of glyphosate at the 4- and 7-leaf growth stage. The various measurements of photosynthesis were reduced by all glyphosate rates, including those recommended for standard weed management. The amount of water used was also reduced by glyphosate treatment. More importantly, water use efficiency, which is the amount of water used for production of plant material, was reduced considerably under glyphosate treatment at all application rates. These results suggest that reduced growth and total plant yields of some GR soybean varieties treated with glyphosate are due to reduced efficiency in converting water into plant material or biomass. The results are useful to other researchers, extension personnel, and farmers by providing an understanding of how glyphosate might interfere with optimum crop productivity. The knowledge derived from this work is essential for pursuing approaches to develop varieties with better water-use efficiency thereby minimizing or eliminating potential adverse effects of glyphosate in GR soybean management systems.

Technical Abstract:
Previous studies comparing cultivars of different maturity groups in different soils demonstrated that early maturity group cultivars were more sensitive to glyphosate injury than those of other maturity groups. In this work, we evaluated the effect of increasing rates of glyphosate on water absorption and photosynthetic parameters in early maturity group cultivar BRS 242 GR soybean. Plants were grown in a complete nutrient solution and subjected to a range of glyphosate rates either as a single or sequential foliar application. Net photosynthesis, transpiration rate, stomatal conductance, sub-stomatal CO2, carboxylation efficiency, fluorescence, maximal fluorescence and chlorophyll content were monitored immediately before and at different growth stages after herbicide application; water absorption was measured daily. All photosynthetic parameters were affected by glyphosate. Total water absorbed and biomass production by plants was also decreased as glyphosate rates increased, with the effect being more intense with a single full rate than half the rate applied in two sequential applications. Water use efficiency (WUE) was significantly reduced with increasing rates of glyphosate. Results suggest that early maturity group GR soybean treated with glyphosate may be more sensitive to drought and less efficient in converting water into biomass compared to GR soybean that do not receive glyphosate.